There is no age restriction. It is up to the parent to decide if their child has the attention span yo sit quietly for the length of the performance, and whether the nature of the performance is appropriate and will hold the child's attention.
You can take children up to age 3 free in "lap seats." However, as Kathryn noted, the maturity and attention span of the child are important. Mowgli was too intense for a 2-year-old seated near us, and our 3-year-old granddaughter just couldn't sit through Diary of a Fourth Grade Nothing. However, our older granddaughter has been transfixed by everything we've seen (as have her grandfather and I).
I think it is a lovely place. However I think the age of a child attending Would vary with the child's maturity and/or - what/who is performing.
I don't know if they have strict rules, and I would say it likely depends on the performance. Some programs are more kid friendly than others. I would call the box office and ask about policies. I would also ask about how long a given performance is and then judge for yourself whether the child in question will enjoy sitting still and being quiet for that long.
I think typically 6 years old, depending on the type of show. You can always call the Box Office.
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